The present invention relates to the art of metal surface treatment. More specifically, the present invention relates to treatment of metal surfaces with a solution of an acid soluble derivative of vegetable tannin to improve the corrosion resistance of the metal.
The need for improvements and protective coatings for metal surfaces to improve the corrosion resistance and paint adhesion characteristics of the metal is well known. Traditionally, metal surfaces are treated with chemicals which form a metal phosphate and/or metal oxide conversion coating on the metal surface to improve the corrosion resistance and paint adhesion thereof. Also traditionally, the conversion coated metal surfaces have been rinsed or post-treated with a solution containing a hexavalent chromium compound for even greater corrosion resistance and paint adhesion.
In recent years, there has been much research and development directed to discovering effective alternatives to the use of hexavalent chromium compounds. The use of tannins in compositions for treatment of bare or corroded metallic surfaces has been disclosed, for example, in U.S. Pat. Nos. 3,975,214, Aug. 17, 1976 to Kulick, et al. and 4,174,980, Nov. 20, 1979 to Howell, Jr., et al. and in references cited therein.
In accordance with the present invention, a novel product which can be made water soluble by partial or complete neutralization with acid is provided by reacting a vegetable tannin such as Quebracho with an aldehyde or formaldehyde and a primary or secondary amine. The product is soluble in water when neutralized with acid or base and is useful for the treatment of aluminum, galvanized metal and other metals for improving their corrosion resistance. Further understanding of this invention will be had from the following disclosure wherein all parts and percentages are by weight unless otherwise indicated.